shunting of oxygenated blood through the ductus arteriosus may present with the rare findings of an upper body that is blue and the lower body pink The need for laboratory evaluation is determined based on the historical features and physical findings established on initial encounter ( Fig 21.1 ) All patients, except very well-appearing newborns and well-appearing cold-exposed patients with peripheral cyanosis only, require measurement of PaO2 Oxygen saturation by pulse oximetry may be helpful in determining if hypoxemia is the cause of cyanosis, but it may also be misleading when abnormal forms of Hb such as methemoglobin or carboxyhemoglobin are present If the PO2 is normal, further laboratory evaluation is determined by the degree of ill appearance Well-appearing cyanotic children with normal PO2 usually have less urgent conditions, such as polycythemia, mild methemoglobinemia, cold exposure, newborn acrocyanosis, or dermatologic findings In this case, laboratory evaluation might include a methemoglobin level and complete blood count (CBC), or no further investigation may be warranted Despite a normal PO2 , an ill-appearing cyanotic patient may have a more emergent condition such as severe methemoglobinemia or septic or cardiogenic shock and may require more aggressive laboratory investigation This might include CBC, co-oximetry including methemoglobin level, blood cultures, and blood chemistry Blood with high methemoglobin content may appear very dark or “chocolate brown” and fails to turn red on exposure to air, such as in a drop on filter paper Methemoglobinemia may improve with intravenous methylene blue If the PO2 is decreased, oxygen therapy should be instituted In general, cyanosis caused by decreased alveolar ventilation or diffusional abnormalities often improves with delivery of 100% O2 However, hypoxemia caused by decreased pulmonary perfusion or shunt will have minimal response to oxygen therapy and can be assessed objectively with the hyperoxia test during which administration of 100% O2 for 10 minutes will increase the PaO2 to over 150 mm Hg in primary pulmonary deficiency but will fail to increase past 100 mm Hg in shunting due to congenital heart disease A chest radiograph should be obtained for evaluation Abnormalities of the lungs may confirm pulmonary disease, and changes in the cardiac size or silhouette may suggest cardiac causes If the chest radiograph is normal, other reasons for diminished PaO2 , such as CNS- or chest wall–related respiratory depression, upper airway obstruction, or pulmonary perfusion abnormalities, must be entertained If a concomitant murmur or other concern for cardiac disease exists, an electrocardiogram (ECG) is essential Abnormal ECGs may suggest cardiac etiologies, either congenital or acquired ( Table 21.1 ) Echocardiography will help establish the definitive diagnosis Suggested Readings and Key References Cortazzo JA, Lichtman AD Methemoglobinemia: a review and recommendations for management J Cardiothorac Vasc Anesth 2014;28(4):1043–1047 Hall JE Principles of gas exchange; diffusion of oxygen and carbon dioxide through the respiratory membrane In: Guyton and Hall Textbook of Medical Physiology 13th ed Philadelphia, PA: Elsevier; 2016:517–526 Lundsgaard C, Van Slyke DD Cyanosis Medicine 1923;2:15 Roche SL, Greenway SC, Redington AN Tetralogy of Fallot with pulmonary stenosis, pulmonary atresia, and absent pulmonary valve In: Allen HD, ed Moss and Adams’ Heart Disease in Infants, Children, and Adolescents: Including the Fetus and Young Adult 9th ed Philadelphia, PA: Wolters Kluwer; 2016:1039–1040 Strobel AM, Lu LN The critically ill infant with congenital heart disease Emerg Med Clin North Am 2015;33(3):501–518 Additional Resources Online e-FIGURE 21.1 Acrocyanosis Note blueness of the hands (Reprinted with permission from Kyle T, Carman S Essentials of Pediatric Nursing 2nd ed Philadelphia, PA: Lippincott Williams & Wilkins; 2012.) e-FIGURE 21.2 Acrocyanosis This commonly appears on the feet and hands of babies shortly after birth This infant is a 32-week newborn (Reprinted with permission from Fletcher M Physical Diagnosis in Neonatology Philadelphia, PA: Lippincott-Raven Publishers; 1998.) ... Physiology 13th ed Philadelphia, PA: Elsevier; 2016:517–526 Lundsgaard C, Van Slyke DD Cyanosis Medicine 1923;2:15 Roche SL, Greenway SC, Redington AN Tetralogy of Fallot with pulmonary stenosis,... Acrocyanosis Note blueness of the hands (Reprinted with permission from Kyle T, Carman S Essentials of Pediatric Nursing 2nd ed Philadelphia, PA: Lippincott Williams & Wilkins; 2012.) e-FIGURE 21.2